1. Field of the Invention
This invention relates generally to thermal printheads which are widely used in facsimile machines and various printers. More specifically, the present invention relates to a thermal printhead of the type which comprises a plurality of unit head substrates arranged in series on a common support member for providing an increased printing width.
2. Description of the Prior Art
As is well known, thermal printheads are widely used to print information on papers of various sizes. Thus, the length of the printhead (namely, the printing width) must be adjusted to suit the particular paper size to which the printhead is applied.
However, if the paper size is too large, it becomes difficult or impractical to increase the length of a single thermal printhead to suit the excessively large paper size such as JIS-A2 (JIS: Japanese Industrial Standard) or larger paper size. Further, it is technically disadvantageous to provide thermal heads of various sizes due to the necessity of redesigning upon every change in size.
In view of these problems, it has been proposed to use a thermal printhead of the type which comprises a plurality of unit head substrates arranged in series on a common support member. Such a printhead, called "divisional-type thermal printhead", enables optional adjustment of the overall printing width by selecting the number of unit head substrates without changing the length of each unit head substrate itself.
However, a prior art divisional-type thermal printhead still has certain problems. For conveniently explaining the problems of the prior art printhead, reference is now made to FIGS. 9 to 18 of the accompanying drawings.
As shown in FIGS. 9 to 14, the prior art divisional-type thermal printhead 10' comprises plural (three for example) unit head substrates 11', 12', 13' arranged in series commonly on an elongate metal support member 14'. The respective unit substrates may adhesively fixed to the support member.
Each of the unit substrates 11'-13' carries a resistor line 11a', 12a', 13a' extending longitudinally of the support member 14' to provide a line of heating dots. The unit substrate has a unit printing width S1', and the printhead 10' as a whole provides an overall printing width S'.
The inner end of the left-hand unit substrate 11' in FIG. 9 is formed with a pair of boundary edges 11b', 11c' which extend from the corresponding resistor line 11a' and are inclined in opposite directions. Similarly, each end of the central unit substrate 12' is formed with a pair of boundary edges 12b', 12c' which extend from the corresponding resistor line 12a' and are inclined in opposite direction. Further, similarly, the inner end of the right-hand unit substrate 13' is formed with a pair of boundary edges 13b', 13c' which extend from the corresponding resistor line 11a' and are inclined in opposite directions. As a result, a pair of V-shaped boundary spaces 15a', 15b' (see FIGS. 9, 10 and 12) are formed between the left-hand and central unit substrates 11', 12', whereas another pair of V-shaped boundary spaces 16a', 16b' (FIG. 9) are formed between the central and right-hand unit substrates 12', 13'.
As shown in FIG. 10, the respective resistor lines 11a'-13a' are displaced transversely from each other by an amount E'. As a result the respective unit substrates 11'-13' can be brought toward each other to make each two adjacent resistor lines overlap each other by an amount W' to provide widthwise linear continuity for printing while still maintaining a minimum clearance T' (see also FIG. 10) of the micrometer order between the adjacent resistor lines.
As shown in FIGS. 15 to 18, the unit head substrates 11'-13' are flanked by respective pairs of connector circuit boards 17a', 17b', 18a', 18b', 19a', 19b'. The pairs of connector circuit boards are held place on the support member 14' by respective pairs of presser covers 22a', 22b', 23a', 23b', 24a', 24b', which are in turn fixed to the support member 14' by respective screws 20', 21'. Though not specifically shown each of the connector circuit boards has a flexible film projecting beyond a backing plate to partially overlap the corresponding unit head substrate.
The respective pairs of presser covers 22a', 22b', 23a', 23b', 24a', 24b', protect arrays of drive ICs 25', 26' arranged on both sides of the resistor lines 11a', 12a', 13a'. Further, the pairs of presser covers also press the respective pairs of connector circuit boards 17a', 17b', 18a', 18b', 19a', 19b' (specifically, the flexible films thereof) into intimate contact with the unit head substrates 11'-13' via respective rubber rods 27', 28'.
In operation of the prior art printhead 10', a platen 29' is pressed downwardly against the printhead in contact with the respectively resistor lines 11a', 12a', 13a', as shown in FIG. 14. Due to such a pressing contact, the support member 14' tends to warp downwardly, as indicated by phantom lines in FIG. 13.
When the support member 14' warps downwardly, the inclined boundary edges 11b', 11c', 12b', 12c', 13b', 13c' of the respective unit head substrates 11'-13' may be brought into mutual contact since the minimum clearance T' is initially very small. As a result, the respective unit head substrates may be damaged (e.g. chipping) at the boundary edges due to such contact, consequently causing a printing quality deterioration at the boundaries between the respective unit head substrates.
Further, in the prior art printhead, the respective boundaries A', B' between the successive presser covers 22a', 22b', 23a', 23b', 24a', 24b' are made to coincide with the boundaries C' between the respective unit head substrates 11'-13' longitudinally of the support plate 14'. Thus, the printhead 10' is most easily bendable at locations where the three kinds of boundaries A', B', C' coincide.
When the platen 29' is pressed downwardly against the printhead 10' having the above-described presser cover assembly (see FIG. 16), the printhead tends to bend abruptly at the boundaries C' between the unit head substrates 11'-13', as indicated by phantom lines in FIG. 17. In this case, obviously the problem of damaging contact between the respective unit head substrates will become more remarkable.